1. Field
This invention relates to an uninterruptible power supply, and more particularly to an uninterruptible power supply that optimizes the DC bus voltage to minimize inrush current during a transfer time between the battery mode and the AC mode.
2. Description of Related Art
The following description sets forth the inventors' knowledge of related art and problems therein and should not be construed as an admission of knowledge in the prior art.
As illustrated in FIG. 1, an uninterruptible power supply (UPS) has a generator 8 that is provided as an alternate source of AC power supply in an event of a power failure. The switching between commercial power 9 and a generator 8 is performed by a switch 10. The UPS system also constitute a battery thyristor 1, a main capacitor module 2, a battery module 3, an input filter circuit 4, an input converter module 5, an output inverter module 6 and an output filter module 7.
As shown in FIG. 2, during switching time from commercial power 9 to generator power 8, the UPS supplies power to the load using the battery module 3. Normally, during this switching time, the voltage across the main capacitor module 2 is boosted up by a battery package using a voltage boost up circuit.
However, as illustrated in FIG. 2, when the UPS is switched from the battery backup mode to the generator mode, the battery thyristor (THY) 1 turns off and the mode is transferred to AC input mode. During this transfer, which may take 1 cycle, the DC voltage across the main capacitor module 2 drops because of the energy consumed in capacitors as shown in by the exponential decay in the graphs of FIGS. 2 and 3. Therefore, when the generator input voltage is applied to the UPS, a large inrush current flows due to the voltage difference between the actual voltage and target voltage, which causes the voltage from the generator to drift causing unstable condition in generator operation as shown by the graphs in FIGS. 2 and 3.
In other words, during switching power from battery module 3 to the generator 8, a high current flows through the UPS due to the voltage drop in the DC bus. Especially, in a case of generator connection to AC line, this current may become large due to oscillations caused by the generator's internal impedance (inductance) and the impedance of input filtering capacitor. As a result of the large oscillatory current, the output voltage of the generator becomes unstable. When the UPS detects this unstable voltage and/or frequency, it moves the UPS to Battery backup mode again. In case where inrush current is too large, the problem of repeated switching between the generator mode and the battery backup mode occur thereby causing battery to burn up additional power.